Writing instrument ink composition and writing instrument containing the same

Abstract

To provide a writing instrument ink composition that has excellent dry-up resistance performance at the writing tip, and can suppress writing defects such as blurring and form good handwriting, and to provide a writing instrument that contains the same.SOLUTION: The writing instrument ink composition is a writing instrument ink composition that contains at least a colorant and a deep eutectic solvent, and, in addition, the deep eutectic solvent contains a hydrogen bond donor compound and a hydrogen bond acceptor compound, the hydrogen bond donor compound is a compound selected from the group consisting of alcohol, saccharide, organic acid and nitrogen-containing compound, and the hydrogen bond acceptor compound is a compound selected from the group consisting of nonmetallic salt, betaine, amino acid, polyvalent carboxylic acid and phosphoric ester, and the writing instrument is a writing instrument that contains said writing instrument ink composition.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present invention relates to an ink composition for a writing instrument and a writing instrument containing the same. More specifically, the present invention relates to an ink composition for a writing instrument that can suppress drying at the writing tip and form good handwriting, and a writing instrument containing the same. [Background technology]

[0002] Conventionally, ink compositions for writing instruments are roughly divided into "water-based ink compositions" that use water as the main solvent, and "oil-based ink compositions" that use an organic solvent such as benzyl alcohol or phenyl glycol as the main solvent. When writing instruments containing these ink compositions are left in the atmosphere with the writing tip exposed, the water or organic solvent evaporates from the writing tip, causing the writing tip to dry out (so-called dry up), which reduces the ink dischargeability and can cause writing defects such as smearing. Therefore, investigations have been conducted into improving the dry-up resistance of ink compositions by blending additives therein (see, for example, Patent Documents 1 and 2).

[0003] Patent Document 1 discloses an aqueous ink composition in which the moisture retention of the writing tip is improved by using a solvent selected from polyhydric alcohols such as ethylene glycol and glycerin.

[0004] Patent Document 2 discloses an oil-based ink composition in which a sucrose fatty acid ester is used to make the writing tip less likely to dry out.

[0005] Although the above-mentioned ink composition has a certain effect of suppressing drying at the writing tip by using a specific compound, the dry-up resistance performance is still insufficient, and it has been difficult to obtain good handwriting without writing defects such as smearing. [Prior art documents] [Patent documents]

[0006] [Patent Document 1] Japanese Patent Application Publication No. 8-209055 [Patent Document 2] JP 2001-200186 A Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention provides an ink composition for a writing instrument which is excellent in resistance to dry-up at the writing tip and exhibits good ink jetting properties, and a writing instrument containing the same. [Means for solving the problem]

[0008] The present invention relates to an ink composition for a writing instrument, which comprises at least a colorant and a deep eutectic solvent. Another requirement is that the deep eutectic solvent comprises a hydrogen bond donor compound and a hydrogen bond acceptor compound, the hydrogen bond donor compound is a compound selected from the group consisting of alcohols, sugars, organic acids, and nitrogen-containing compounds, the hydrogen bond acceptor compound is a compound selected from the group consisting of non-metal salts, betaines, amino acids, polycarboxylic acids, and phosphate esters, and further contains water. A further feature is a writing instrument containing the ink composition. Another requirement is that the writing instrument is a retractable writing instrument. Effect of the Invention

[0009] The present invention can provide an ink composition for a writing instrument that has excellent resistance to drying up at the writing tip and can form good handwriting by suppressing writing defects such as smearing, and a writing instrument containing the same. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The ink composition for a writing instrument according to the present invention (hereinafter, sometimes referred to as "ink composition" or "ink") contains at least a colorant and a deep eutectic solvent. Each component constituting the ink composition according to the present invention will be described below.

[0011] The colorant applied to the present invention is not particularly limited, and may be any dye or pigment that is soluble or dispersible in the deep eutectic solvent described below.

[0012] The dyes include acid dyes, basic dyes, direct dyes, oil-soluble dyes, disperse dyes, and the like.

[0013] Examples of acid dyes include New Coccine (CI16255), Tartrazine (CI19140), Acid Blue Black 10B (CI20470), Guinea Green (CI42085), Brilliant Blue FCF (CI42090), Acid Violet 6B (CI42640), Soluble Blue (CI42755), Naphthalene Green (CI44025), Eosine (CI45380), Phloxine (CI45410), Erythrosine (CI45430), Nigrosine (CI50420), and Acid Flavin (CI56205).

[0014] Examples of basic dyes include chrysoidine (CI11270), methyl violet FN (CI42535), crystal violet (CI42555), malachite green (CI42000), Victoria blue FB (CI44045), rhodamine B (CI45170), acridine orange NS (CI46005), and methylene blue B (CI52015).

[0015] Examples of direct dyes include Congo Red (CI22120), Direct Sky Blue 5B (CI24400), Violet BB (CI27905), Direct Deep Black EX (CI30235), Kayalas Black G Conc (CI35225), Direct Fast Black G (CI35255), and Phthalocyanine Blue (CI74180).

[0016] Examples of oil-soluble dyes include CI Solvent Black 7, CI Solvent Black 123, CI Solvent Blue 2, CI Solvent Blue 25, CI Solvent Blue 55, CI Solvent Blue 70, CI Solvent Red 8, CI Solvent Red 49, CI Solvent Red 100, CI Solvent Violet 8, CI Solvent Violet 21, CI Solvent Green 3, CI Solvent Yellow 21, CI Solvent Yellow 44, CI Solvent Yellow 61, and CI Solvent Orange 37.

[0017] Examples of disperse dyes include CI Disperse Yellow 82, CI Disperse Yellow 3, CI Disperse Yellow 54, CI Disperse Red 191, CI Disperse Red 60, and CI Disperse Violet 57.

[0018] Examples of the pigment include inorganic pigments, organic pigments, glittering pigments, fluorescent pigments, and phosphorescent pigments.

[0019] Examples of inorganic pigments include carbon black, titanium oxide, iron oxide black, yellow iron oxide, red iron oxide, and ultramarine.

[0020] Examples of organic pigments include azo pigments, phthalocyanine pigments, quinacridone pigments, perylene pigments, perinone pigments, isoindolinone pigments, isoindoline pigments, dioxazine pigments, thioindigo pigments, anthraquinone pigments, quinophthalone pigments, diketopyrrolopyrrole pigments, threne pigments, indigo pigments, phthalone pigments, methine azomethine pigments, and metal complex pigments.

[0021] As the pigment, a water-dispersed pigment, which is a pigment finely and stably dispersed in an aqueous medium in advance using a surfactant or a resin, can also be used.

[0022] Examples of resins for dispersing pigments include polyamides, urethane resins, polyesters, epoxy resins, melamine resins, phenolic resins, silicone resins, polyvinyl alcohol, polyvinylpyrrolidone, polyvinyl acetate, polyvinyl chloride, polyvinylidene chloride, polystyrene, acrylic acid resins, maleic acid resins, gum arabic, cellulose, dextran, casein, and derivatives thereof, copolymers of the above-mentioned resins, and the like.

[0023] Examples of luster pigments include metallic luster pigments in which the surface of a core substance such as a piece of glass is coated with gold, silver, or the like; pearl pigments in which the surface of a core substance such as natural mica, synthetic mica, or flaky aluminum oxide is coated with a metal oxide such as titanium oxide; cholesteric liquid crystal pigments; metal powder pigments; metal pigments obtained by peeling off a vapor-deposited film of metal such as aluminum formed on a substrate such as a film; and metal pigments in which a vapor-deposited film of metal such as aluminum is formed on a colorless, transparent or colored transparent film and then powdered.

[0024] Examples of fluorescent pigments include fine synthetic resin particles in which various fluorescent dyes are dissolved in a resin matrix.

[0025] Any general-purpose phosphorescent pigment can be used as long as it has the property of absorbing and storing light from the sun or an electric lamp, and gradually releasing and emitting light in a dark place (this is called afterglow). Examples of phosphorescent pigments include CaS / Bi-based, CaSrS / Bi-based, ZnS / Cu-based, ZnCdS / Cu-based, and SrAl2O4 / rare earth metal-based pigments.

[0026] The colorants can be used alone or in combination of two or more.

[0027] When the above-mentioned pigments are used as colorants, pigment dispersants can be used as necessary. Examples of pigment dispersants include anionic and nonionic surfactants, anionic polymers such as polyacrylic acid and styrene-acrylic acid, and nonionic polymers such as PVP and PVA.

[0028] The dyes or pigments described above can be used as they are, but microcapsule pigments in which the dyes or pigments are encapsulated in microcapsules, and resin particles containing the dyes or pigments can also be used as colorants. In particular, by encapsulating the dyes or pigments in microcapsules, they can be isolated and protected from the external environment, and the water resistance and light resistance of the encapsulated matter can be improved.

[0029] The microcapsule pigment can be formed by encapsulating in microcapsules a colored body in which the above-mentioned dye or pigment is dissolved or dispersed in an oil medium.

[0030] Examples of oil-based media include esters such as monobasic acid esters, dibasic acid monoesters, dibasic acid diesters, partial esters or complete esters of polyhydric alcohols, aromatic hydrocarbons such as alkylbenzenes and alkylnaphthalenes, higher alcohols, ketones, ethers, and the like. The oil medium may be used alone or in combination of two or more.

[0031] Microencapsulation of the microcapsule pigment can be appropriately selected according to the application from the conventionally known isocyanate-based interfacial polymerization method, melamine-formaldehyde-based in situ polymerization method, liquid curing coating method, phase separation method from an aqueous solution, phase separation method from an organic solvent, melt dispersion cooling method, air suspension coating method, spray drying method, etc. Examples of the capsule material include epoxy resin, urea resin, urethane resin, isocyanate resin, etc.

[0032] Depending on the purpose, a secondary resin film may be further provided on the surface of the microcapsules to impart durability or modify the surface properties for practical use.

[0033] Examples of the resin particles containing a dye include resin particles in which the above-mentioned dye is homogeneously dissolved or dispersed in the resin particles, and resin particles dyed with a dye.

[0034] The resin constituting the resin particles is not particularly limited, but a thermosetting resin is preferred. Thermosetting resins are preferable because they have superior solvent resistance and heat resistance compared to thermoplastic resins, and also have superior resistance to migration of the dye contained therein, and can prevent the dye from eluting from the resin. Among the thermosetting resins, guanamine resin or melamine resin is preferred since it is possible to further suppress the elution of the dye.

[0035] Examples of the resin particles containing a pigment include resin particles in which the above-mentioned pigment is uniformly dispersed in the resin particles, and resin particles whose surfaces are coated with a pigment.

[0036] The resin constituting the resin particles is not particularly limited, and a general-purpose resin can be used.

[0037] The resin particles can be produced by a pulverization method, a spray drying method, or a polymerization method in which polymerization is carried out in an aqueous or oily medium in the presence of a dye or pigment, such as a suspension polymerization method, a suspension polycondensation method, a dispersion polymerization method, or an emulsion polymerization method.

[0038] The shape of the resin particles is not particularly limited, and resin particles having a spherical shape such as a perfect sphere, an oval sphere, or an approximately spherical shape, a polygonal shape, a flat shape, etc., can be used. Among these, it is preferable to use spherical resin particles.

[0039] When the colorant is a dye or pigment, or the above-mentioned microencapsulated pigment or resin particles, the content of the colorant relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 0.01 to 50 mass%, more preferably 0.1 to 30 mass%, and even more preferably 1 to 20 mass%. If the content of the colorant exceeds 50 mass%, the ink dischargeability of the writing instrument containing the ink composition is likely to decrease, and writing defects such as smearing and skipping of lines are likely to occur. On the other hand, if the content is less than 0.01 mass%, it is difficult to obtain a suitable writing density for the writing instrument.

[0040] As the colorant, a thermochromic material that changes color with temperature change, a photochromic material that changes color with light irradiation, or other color-changing materials can be used. These color changes may be reversible or irreversible. Reversible thermochromic materials and reversible photochromic materials are preferred because they can repeatedly change color with temperature change or light irradiation.

[0041] Examples of reversible thermochromic materials include a reversible thermochromic composition comprising (a) an electron-donating color-forming organic compound, (b) an electron-accepting compound, and (c) a reaction medium that determines the temperature at which the color-forming reaction of the above components (a) and (b) occurs.

[0042] As the reversible thermochromic composition, a reversible thermochromic composition of the heat-discoloring type having a relatively small hysteresis width (ΔH) (ΔH=1 to 7°C) described in JP-B-51-44706, JP-B-51-44707, JP-B-1-29398, etc. can be used. The heat-discoloring type means that the composition is decolored by heating and colored by cooling. This reversible thermochromic composition discolors around a predetermined temperature (discoloration point), and exhibits a discolored state in a temperature range above the high-temperature discoloration point and a colored state in a temperature range below the low-temperature discoloration point. Of the two states, only one specific state exists in the room temperature range, and the other state is maintained while the heat or cold required to manifest that state is applied, but returns to the state exhibited in the room temperature range when the application of heat or cold is removed.

[0043] As the reversible thermochromic composition, a heat-discolorable reversible thermochromic composition having a large hysteresis width (ΔH=8 to 80° C.) described in JP-B-4-17154, JP-A-7-179777, JP-A-7-33997, JP-A-8-39936, JP-A-2005-1369, etc. can be used. The heat-discolorable type means that the color disappears when heated and the color appears when cooled. This reversible thermochromic composition changes color along a path that is significantly different when the temperature is increased from a lower temperature side than the discoloration temperature range than when the temperature is decreased from a higher temperature side than the discoloration temperature range, and has color memory in a specific temperature range [the temperature range between the color development onset temperature t2 and the discoloration onset temperature t3 (temperature range in which two phases are essentially maintained)] in which the colored state is in a temperature range below the complete color development temperature t1, or the discolored state is in a high temperature range above the complete discoloration temperature t4.

[0044] In addition, when the reversible thermochromic composition having the above-mentioned color memory property is applied to the present invention, the reversible thermochromic composition can effectively function to retain the color exhibited under normal conditions (daily living temperature range) by specifying the complete color development temperature t1 to a temperature that can only be obtained in a freezer or cold region, and the complete decolorization temperature t4 to a temperature range that can be obtained from frictional heat produced by a friction body or a familiar heating body such as a hair dryer, and specifying the ΔH value to be 40 to 100°C.

[0045] The temperature that can only be obtained in freezers, cold regions, etc. is in the range of -50 to 0°C, preferably -40 to -5°C, and more preferably -30 to -10°C. The temperature obtainable from a common heating element such as a hair dryer is in the range of 50 to 95°C, preferably 50 to 90°C, and more preferably 60 to 80°C.

[0046] As the reversible thermochromic composition, a heat-coloring type reversible thermochromic composition using a gallic acid ester, as described in JP-B-51-44706, JP-A-2003-253149, etc., can also be used. The heat-coloring type means that the color develops when heated and the color disappears when cooled.

[0047] The reversible thermochromic composition is a compatible solution containing the above components (A), (B), and (C) as essential components, and the ratio of each component depends on the concentration, color change temperature, color change form, and type of each component. In general, the component ratio that provides the desired properties is in the range of 1:1 for component (A) to 0.1-100, preferably 0.1-50, more preferably 0.5-20 for component (B), and 1-800, preferably 5-200, more preferably 10-100 for component (C) (all the above ratios are in parts by mass).

[0048] Examples of reversible photochromic materials include photochromic compounds such as conventionally known spirooxazine derivatives, spiropyran derivatives, and naphthopyran derivatives that develop color when irradiated with sunlight, ultraviolet light, or blue light with a peak emission wavelength in the range of 400 to 495 nm, and lose color when the irradiation is stopped.

[0049] Examples of spirooxazine derivatives include conventionally known indolinospirobenzoxazine compounds, indolinospironaphthoxazine compounds, indolinospirophenanthrooxazine compounds, indolinospiroquinolinoxazine compounds, and the like.

[0050] Furthermore, examples of photochromic compounds having a photomemory property (color memory photochromic property) include conventionally known fulgide derivatives and diarylethene derivatives.

[0051] Furthermore, as the reversible photochromic material, a reversible photochromic composition in which the above-mentioned photochromic compound is dissolved in various oligomers can also be used. Examples of the oligomer include styrene-based oligomers, acrylic-based oligomers, terpene-based oligomers, and terpene-phenol-based oligomers.

[0052] Examples of styrene oligomers include low molecular weight polystyrene, styrene-α-methylstyrene copolymer, α-methylstyrene polymer, α-methylstyrene-vinyltoluene copolymer, and the like.

[0053] The acrylic oligomer may, for example, be an acrylic acid ester copolymer.

[0054] Examples of the terpene oligomer include α-pinene polymer, β-pinene polymer, d-limonene polymer, and the like.

[0055] An example of the terpene phenol oligomer is an α-pinene-phenol copolymer.

[0056] By dissolving the photochromic compound in various oligomers, it is possible to improve the light resistance of the photochromic compound, as well as to improve the color development density and adjust the color change sensitivity.

[0057] The oligomers can be used alone or in combination of two or more.

[0058] The above-mentioned reversible thermochromic composition or reversible photochromic composition is effective when used as it is, but it can also be encapsulated in a microcapsule to form a reversible thermochromic microcapsule pigment or a reversible photochromic microcapsule pigment, or dispersed in a thermoplastic resin or a thermosetting resin to form reversible thermochromic resin particles or reversible photochromic resin particles, and used as a colorant to be applied to the present invention. In the following, the reversible thermochromic microencapsulated pigment and the reversible photochromic microencapsulated pigment may be referred to as "microencapsulated pigment", and the reversible thermochromic resin particles and the reversible photochromic resin particles may be referred to as "resin particles".

[0059] The reversible thermochromic composition or the reversible photochromic composition is preferably encapsulated in a microcapsule to form a reversible thermochromic microcapsule pigment or a reversible photochromic microcapsule pigment. This is because by encapsulating in a microcapsule, a chemically and physically stable microcapsule pigment can be formed, and further, the reversible thermochromic composition or the reversible photochromic composition can be maintained in the same composition under various conditions of use, and can exert the same action and effect.

[0060] Microencapsulation may be appropriately selected according to the application from the conventionally known isocyanate-based interfacial polymerization method, melamine-formaldehyde-based in situ polymerization method, liquid curing coating method, phase separation method from an aqueous solution, phase separation method from an organic solvent, melt dispersion cooling method, air suspension coating method, spray drying method, etc. Examples of capsule materials include epoxy resins, urea resins, urethane resins, isocyanate resins, etc.

[0061] Depending on the purpose, a secondary resin film may be further provided on the surface of the microcapsules to impart durability or modify the surface properties for practical use.

[0062] The above-mentioned microcapsule pigment preferably has a mass ratio of inclusions to wall film of 7:1 to 1:1, and by having the mass ratio of inclusions to wall film within the above range, it is possible to prevent a decrease in color density and clarity during color development. More preferably, the mass ratio of inclusions to wall film is 6:1 to 1:1.

[0063] By incorporating a non-color-changing colorant such as a general dye or pigment into the microencapsulated pigment, it is possible to produce a microencapsulated pigment that exhibits color-changing behavior from color (1) to color (2).

[0064] When the colorant is the above-mentioned discoloration material, the content of the colorant relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 5 to 40 mass%, more preferably 10 to 40 mass%, and even more preferably 15 to 35 mass%. If the content of the colorant exceeds 40 mass%, the ink dischargeability of the writing instrument containing the ink composition decreases, and writing defects such as smearing and skipping of lines tend to occur. On the other hand, if the content is less than 5 mass%, it is difficult to obtain suitable discoloration and writing density as a writing instrument, and it is difficult to fully satisfy the discoloration function.

[0065] The average particle size of the reversible thermochromic microencapsulated pigment or resin particles, or the reversible photochromic microencapsulated pigment or resin particles is preferably in the range of 0.01 to 5 μm, more preferably 0.1 to 3 μm, and even more preferably 0.5 to 3 μm. If the average particle size of the microencapsulated pigment or resin particles exceeds 5 μm, it becomes difficult to obtain good ink dischargeability when used in a writing instrument. On the other hand, if the average particle size is less than 0.01 μm, it becomes difficult to exhibit high-concentration color development.

[0066] The average particle size was measured by determining the particle region using image analysis particle size distribution measurement software (manufactured by Mountec Co., Ltd., product name: MacView), calculating the projected area equivalent circle diameter (Heywood diameter) from the area of ​​the particle region, and measuring the average particle size of particles equivalent to a sphere of equal volume using this value.

[0067] In addition, when the particle size of all or the majority of the particles exceeds 0.2 μm, it is also possible to measure the average particle size of particles equivalent to an equal volume sphere by the Coulter method using a particle size distribution analyzer (product name: Multisizer 4e, manufactured by Beckman Coulter, Inc.).

[0068] Furthermore, the volumetric particle size and average particle size may be measured using a calibrated laser diffraction / scattering particle size distribution analyzer (manufactured by HORIBA, Ltd., product name: LA-300) based on values ​​measured using the above-mentioned software or a measuring device using the Coulter method.

[0069] Deep eutectic solvents (DESs) applied to the present invention are compounds that contain a hydrogen bond donor compound (hereinafter sometimes referred to as "hydrogen bond donor") and a hydrogen bond acceptor compound (hereinafter sometimes referred to as "hydrogen bond acceptor"), and are liquid at room temperature, formed by mixing the hydrogen bond donor and the hydrogen bond acceptor in a certain ratio. Here, both or either one of the hydrogen bond donor and the hydrogen bond acceptor is solid at room temperature (e.g., 25°C). Room temperature is a temperature range of 20°C ± 15°C (5 to 35°C) as specified by JIS Z8703.

[0070] Deep eutectic solvents are liquid at room temperature. This is due to the eutectic melting point depression that occurs when a hydrogen bond donor compound and a hydrogen bond acceptor compound are mixed. The melting point of the mixture prepared by each compound is significantly lower than the melting point of each compound, even if at least one of the hydrogen bond donor and hydrogen bond acceptor is solid at room temperature, so the deep eutectic solvent is in a liquid state at room temperature.

[0071] Deep eutectic solvents are characterized by low vapor pressure and non-volatility. In general, when an ink composition is stored in a writing instrument, particularly a retractable writing instrument, the writing tip (hereinafter sometimes referred to as the "pen tip") is placed in an environment where it is prone to drying, which can easily cause writing defects such as smearing, and can even make writing impossible. The state in which the pen tip is dry is called "dry-up." The deep eutectic solvent has the effect of suppressing dry-up in the ink for writing instruments according to the present invention, and improves the ink discharge properties from the writing tip. In other words, an ink composition using a deep eutectic solvent has excellent resistance to drying up and inhibits drying at the writing tip, thereby achieving excellent writing performance.

[0072] The hydrogen bond donor compound is not particularly limited as long as it forms a deep eutectic solvent when mixed with a hydrogen bond acceptor compound.

[0073] Examples of hydrogen bond donor compounds include metal compounds such as halides of metal elements (hereinafter sometimes referred to as "metal halides").

[0074] Examples of metal elements constituting the metal halide include lithium, aluminum, chromium, iron, cobalt, nickel, copper, zinc, gallium, yttrium, silver, cadmium, indium, tin, and lanthanum.

[0075] Examples of halogen atoms constituting the metal halide include a fluorine atom, a chlorine atom, a bromine atom, and an iodine atom, and preferably a chlorine atom or a bromine atom.

[0076] Examples of metal halides include lithium chloride, aluminum chloride, iron(III) chloride, copper(I) chloride, copper(II) chloride, zinc(II) chloride, gallium(III) chloride, yttrium(III) chloride, silver chloride, cadmium chloride, zirconium(III) chloride, indium(III) chloride, tin(II) chloride, tin(IV) chloride, and lanthanum(III) chloride. The metal halides can be used alone or in combination of two or more.

[0077] The metal halide may be a hydrate, and examples of the hydrate of the metal halide include chromium(III) chloride hexahydrate, cobalt(II) chloride hexahydrate, copper(II) chloride dihydrate, nickel(II) chloride hexahydrate, and iron(III) chloride hexahydrate.

[0078] As the hydrogen bond donor compound, non-metallic compounds such as alcohols, sugars, organic acids, and nitrogen-containing compounds can also be used.

[0079] Alcohols are organic compounds that contain a hydroxyl group (-OH) and include mono-, di-, and triols as well as sugar alcohols. Examples of alcohols include ethylene glycol, triethylene glycol, hexanediol, 1,4-butanediol, glycerin, mannitol, sorbitol, inositol, isosorbide, xylitol, ribitol, galactitol, erythritol, maltitol, arabitol, resorcinol, pentaerythritol, dipentaerythritol, and adonitol. The alcohols can be used alone or in combination of two or more.

[0080] Sugars include not only monosaccharides and disaccharides but also oligosaccharides. Examples of sugars include sucrose, glucose, fructose, lactose, maltose, cellobiose, arabinose, ribose, ribulose, galactose, rhamnose, raffinose, xylose, mannose, trehalose, sucrose, glucose, sucrose, glucuronic acid, sialic acid, and the like. The sugars can be used alone or in combination of two or more.

[0081] Organic acids are organic compounds that contain a carboxy group (-COOH) and include monocarboxylic acids, dicarboxylic acids, tricarboxylic acids, and their salts. Organic acids include lactic acid, tartaric acid, malic acid, ascorbic acid, citric acid, stearic acid, adipic acid, oleic acid, isberic acid, linoleic acid, maleic acid, pyruvic acid, fumaric acid, malonic acid, oxalic acid, succinic acid, 1,2,3-propanetricarboxylic acid, levulinic acid, itaconic acid, suberic acid, formic acid, acetic acid, butanoic acid, pentanoic acid, hexanoic acid, heptanoic acid, octanoic acid, nonanoic acid, decanoic acid, dodecanoic acid, tetradecanoic acid, pentadecanoic acid, and tetradecanoic acid. Examples of the carboxylic acids include aliphatic carboxylic acids such as decanoic acid, hexadecanoic acid, heptadecanoic acid, octadecanoic acid, eicosanoic acid, docosanoic acid, tetracosanoic acid, hexacosanoic acid, octacosanoic acid, triacontanoic acid, glycolic acid, glutaric acid, aconitic acid, and neuraminic acid; and aromatic carboxylic acids such as benzoic acid, phenylacetic acid, 3-phenylpropionic acid, 4-hydroxybenzoic acid, caffeic acid, p-coumaric acid, trans-cinnamic acid, and gallic acid. The organic acids can be used alone or in combination of two or more.

[0082] Examples of the nitrogen-containing compound include urea compounds, amide compounds, azole compounds, and pyridine compounds.

[0083] Examples of the urea compound include urea, 1-methylurea, 1,3-dimethylurea, 1,1-dimethylurea, ethyleneurea, propyleneurea, tetramethylurea, trifluoromethylurea, and thiourea.

[0084] Examples of the amide compound include acetamide, methylacetamide, dimethylacetamide, benzamide, and 2,2,2-trifluoroacetamide.

[0085] Examples of the azole compound include pyrazole, imidazole, benzimidazole, imidazoline, thiazole, and oxazole.

[0086] Examples of the pyridine compound include 4-propylpyridine and 2-hydroxypyridine.

[0087] The nitrogen-containing compounds can be used alone or in combination of two or more.

[0088] As the hydrogen bond donor compound, a compound selected from the group consisting of non-metallic compounds such as alcohols, sugars, organic acids, and nitrogen-containing compounds is preferred because of its excellent handleability, and as the nitrogen-containing compound, a urea compound is preferred.

[0089] The hydrogen bond acceptor compound is not particularly limited as long as it forms a deep eutectic solvent when mixed with a hydrogen bond donor compound.

[0090] Examples of the hydrogen bond acceptor compound include salts, betaines, amino acids, polycarboxylic acids, and phosphates.

[0091] Examples of salts include salts of metal elements (hereinafter sometimes referred to as "metal salts") and salts of non-metal elements (hereinafter sometimes referred to as "non-metal salts").

[0092] Examples of metal salts include metal compounds such as halides of metal elements (hereinafter sometimes referred to as "metal halides"), metal carbonates, metal nitrates, and metal phosphates.

[0093] Examples of metal halides include sodium chloride, magnesium chloride, aluminum chloride, aluminum bromide, potassium chloride, iron(III) chloride, copper(II) chloride, zinc(II) chloride, zinc(II) bromide, zirconium(III) chloride, and tin(II) chloride.

[0094] Examples of metal carbonates include sodium hydrogen carbonate and potassium carbonate. Examples of metal nitrates include sodium nitrate. Examples of metal phosphates include sodium dihydrogen phosphate.

[0095] The metal salts can be used alone or in combination of two or more.

[0096] Examples of non-metallic salts include ammonium salts, amine hydrochlorides, and phosphonium salts.

[0097] Examples of ammonium salts include choline chloride, choline fluoride, ethylammonium chloride, tetrabutylammonium chloride, tetrabutylammonium bromide, tetrapropylammonium chloride, tetramethylammonium chloride, methyltrioctylammonium chloride, tetraoctylammonium chloride, acetylcholine chloride, chlorocholine chloride, fluorocholine bromide, fluorocholine chromide, benzyltriethylammonium chloride, tetraethylammonium bromide, bis(2-hydroxyethyl)diethylammonium chloride, N-(2-hydroxyethyl)-N,N-dimethylbenzenemethanaminium chloride, 2-(chlorocarbonyloxy)-N,N,N-trimethylethane ammonium chloride, N,N-diethylethanol ammonium chloride, trimethylphenylammonium chloride, 1-butyl-3-methylimidazolium chloride, choline nitrate, choline tetrafluoroborate, and other quaternary ammonium salts.

[0098] Examples of amine hydrochlorides include 2-(diethylamino)ethanol hydrochloride, ethylamine hydrochloride, and guanidine hydrochloride.

[0099] Examples of the phosphonium salt include quaternary phosphonium salts such as methyltriphenylphosphonium bromide, benzyltriphenylphosphonium chloride, tetrabutylphosphonium bromide, and allyltriphenylphosphonium bromide.

[0100] The nonmetallic salts can be used alone or in combination of two or more.

[0101] Betaines are generally zwitterions, and are compounds that have a cationic functional group such as a quaternary ammonium cation or a quaternary phosphonium cation, and an anionic functional group such as a carboxyl group or a sulfo group, and do not have an electric charge in the molecule as a whole. Examples of betaines include trimethylglycine. Betaines can be used alone or in combination of two or more.

[0102] The amino acids may be not only naturally occurring amino acids but also artificial amino acids (non-natural amino acids), and include α-, β-, γ-, and δ-amino acids. Examples of amino acids include γ-aminobutyric acid, alanine, glutamic acid, aspartic acid, asparagine, lysine, arginine, serine, glycine, proline, threonine, carnitine, and cysteine. The amino acids can be used alone or in combination of two or more.

[0103] Examples of polycarboxylic acids include malic acid and citric acid. The polyvalent carboxylic acids can be used alone or in combination of two or more.

[0104] Examples of the phosphate ester include triethyl phosphate and triphenyl phosphate. The phosphate esters can be used alone or in combination of two or more.

[0105] As the hydrogen bond acceptor compound, a compound selected from the group consisting of nonmetallic compounds, nonmetallic salts, betaines, amino acids, polycarboxylic acids, and phosphate esters is preferred because of its excellent handleability, and as the nonmetallic salts, quaternary ammonium salts and quaternary phosphonium salts are preferred.

[0106] Ionic liquids (ILs) have been used as materials that exhibit non-volatility like deep eutectic solvents. Deep eutectic solvents are known to have similar characteristics to ionic liquids, but deep eutectic solvents are completely different from ionic liquids. Ionic liquids are materials that are defined as "liquids consisting only of ions," "liquid electrolytes consisting of 100% ions," "completely consisting of ions," etc. On the other hand, as mentioned above, deep eutectic solvents are "composed of hydrogen bond donor compounds and hydrogen bond acceptor compounds." In other words, deep eutectic solvents are not ionic liquids because they are not composed only of ions in that they contain hydrogen bond donor compounds. Deep eutectic solvents are also non-volatile and have advantages over ionic liquids.

[0107] Deep eutectic solvents can be prepared by simply mixing hydrogen bond donors and hydrogen bond acceptors without chemical reactions. Furthermore, hydrogen bond donor and hydrogen bond acceptor compounds are easily available and relatively inexpensive, so they are considered to be less expensive than ionic liquids.

[0108] Since many hydrogen bond donor and hydrogen bond acceptor compounds have a small impact on the environment, deep eutectic solvents are thought to pose a lower environmental burden when disposed of than ionic liquids.

[0109] In the deep eutectic solvent, the hydrogen bond donor compound is preferably a compound selected from the group consisting of alcohols, sugars, organic acids, and nitrogen-containing compounds, and the hydrogen bond acceptor compound is preferably a compound selected from the group consisting of nonmetal salts, betaines, amino acids, polycarboxylic acids, and phosphate esters. A deep eutectic solvent made with this combination is preferable because it does not contain metal salts, is easy to handle, and is considered to have a small impact on the environment. Furthermore, it is more preferable that the hydrogen bond donor compound is a compound selected from the group consisting of alcohols, organic acids, and urea compounds, and the hydrogen bond acceptor compound is a compound selected from the group consisting of quaternary ammonium salts and quaternary phosphonium salts. Deep eutectic solvents made from this combination are more suitable because many of them are biodegradable and can be prepared at a lower cost.

[0110] The method for producing the deep eutectic solvent is not particularly limited, and any conventionally known method can be used. Specifically, it can be produced by directly mixing a hydrogen bond donor compound and a hydrogen bond acceptor compound. In this case, the mixture prepared by mixing each compound may be heated to melt, and then cooled to room temperature to produce a completely homogenized deep eutectic solvent. In addition, when each compound is solid at room temperature (e.g., 25°C), it can also be produced by heating one compound (preferably the compound with the lower melting point of the two compounds) to melt it, and then mixing or dissolving the other compound.

[0111] When each compound is solid at room temperature, the hydrogen bond donor compound and the hydrogen bond acceptor compound may be directly mixed together, and at least one of water and glycerin may be added. Here, the amount of water or glycerin added is small. Specifically, the amount of water or glycerin is an amount insufficient to completely dissolve each compound that is solid at room temperature, or an amount that simultaneously saturates each compound that is solid at room temperature. Even in such a case, the mixture becomes liquid at room temperature, and a deep eutectic solvent can be obtained.

[0112] A specific example of a deep eutectic solvent in which the amount of water or glycerin is insufficient to completely dissolve each compound that is solid at room temperature is 149 g of sorbitol (hydrogen bond donor), 47 g of alanine (hydrogen bond acceptor), and 60 g of water. At 25°C, only about 33 g of alanine dissolves in 60 g of water, and therefore 60 g of water is insufficient to completely dissolve alanine. However, by mixing sorbitol into this, it can be made into a liquid and can be used as a deep eutectic solvent. The solubility of each compound is evaluated at 25°C.

[0113] A specific example of a deep eutectic solvent in which the amount of water or glycerin is such that each compound that is solid at room temperature is saturated at the same time is 73 g of fructose (hydrogen bond donor), 7.2 g of sodium chloride (hydrogen bond acceptor), and 20 g of water. At 25°C, 73 g of fructose is saturated with 20 g of water, and when 73 g of sodium chloride is mixed therewith, each compound is saturated with water at the same time, so that it can be used as a deep eutectic solvent. In addition, at 25°C, 7.2 g of sodium chloride is saturated with 20 g of water, but even if 73 g of fructose is mixed therewith, it can be used as a deep eutectic solvent in the same way. That is, sodium chloride and fructose are saturated with water at the same time in the deep eutectic solvent at 25°C, so that it can be used as a deep eutectic solvent. The solubility of each compound is evaluated at 25°C.

[0114] The content of at least one of water and glycerin relative to the deep eutectic solvent is preferably in the range of 0 to 50 mass %, more preferably 0 to 30 mass %, and further preferably 0 to 20 mass %.

[0115] The deep eutectic solvents can be used alone or in combination of two or more.

[0116] In the ink composition according to the present invention, the deep eutectic solvent is preferably the main solvent. Here, the "main solvent" in the present invention means a solvent whose content relative to the total mass of the ink composition is 50 mass% or more. An ink composition using a deep eutectic solvent as the main solvent uses a new solvent different from conventional water-based ink compositions and oil-based ink compositions, and can be an ink composition with excellent dry-up resistance without blending additives. When the deep eutectic solvent is the main solvent, the content of the deep eutectic solvent relative to the total mass of the ink composition is preferably 60 mass % or more, more preferably 70 mass % or more, and even more preferably 80 mass % or more.

[0117] When the ink composition according to the present invention is an ink composition using a deep eutectic solvent as the main solvent, it may further contain at least one of water and a water-soluble organic solvent. By blending at least one of water and a water-soluble organic solvent, the viscosity of the ink composition can be reduced, and the ink discharge property from the writing tip can be improved, thereby improving the writing feeling.

[0118] Examples of water include tap water, ion-exchanged water, ultrafiltered water, and distilled water.

[0119] Examples of the water-soluble organic solvent include ethanol, propanol, butanol, glycerin, sorbitol, triethanolamine, diethanolamine, monoethanolamine, ethylene glycol, diethylene glycol, thioethylene glycol, polyethylene glycol, propylene glycol, butylene glycol, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, propylene glycol monobutyl ether, ethylene glycol monomethyl ether acetate, sulfolane, 2-pyrrolidone, and N-methyl-2-pyrrolidone. The water-soluble organic solvents can be used alone or in combination of two or more kinds.

[0120] When the ink composition according to the present invention contains at least one of water and a water-soluble organic solvent, the content of at least one of water and a water-soluble organic solvent relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 1 to 40 mass%, more preferably 5 to 30 mass%, and even more preferably 10 to 25 mass%.

[0121] The ink composition according to the present invention may be an aqueous ink composition (aqueous ink) containing water as the main solvent. Here, the "aqueous ink composition" in the present invention refers to an ink composition in which the content of water relative to the total mass of the ink composition is 50 mass% or more. By blending a deep eutectic solvent in the aqueous ink composition, it is possible to suppress the evaporation of water from the writing tip and improve the dry-up resistance of the aqueous ink composition.

[0122] When water is the main solvent, the content of water relative to the total mass of the ink composition is preferably in the range of 70 mass % or more, more preferably 80 mass % or more, and even more preferably 90 mass % or more. Furthermore, the content of the deep eutectic solvent relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 1 to 20 mass %, more preferably 3 to 15 mass %.

[0123] The ink composition according to the present invention can be blended with a thickener to provide an ink composition having good stability over time. As the thickener, any conventionally known substance can be used, but it is preferable to use a substance that can impart shear thinning properties to the ink composition (shear thinning agent).

[0124] An ink composition using a shear thinning agent has high viscosity and is difficult to flow when left at rest or under low stress, but easily becomes less viscous when external stress is applied. This makes it possible to prevent ink leakage, separation, and backflow when not writing, and makes it easy to improve the ink ejection stability from the pen tip when writing.

[0125] In particular, when such an ink composition is used in a writing instrument (ballpoint pen) equipped with a ballpoint tip as the pen tip, the ink composition is stably held in the ballpoint pen because the ink composition has a high viscosity when left stationary without shear stress. Therefore, when writing, a strong shear stress is applied to the ink composition due to the rotation of the ball, and the ink composition in the vicinity of the ball is more likely to have a low viscosity, which can improve the ink discharge stability.

[0126] When the ink composition according to the present invention contains a thickener, the content of the thickener relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 0.1 to 20 mass %.

[0127] Examples of shear thinning agents include water-soluble polysaccharides, polymers having a molecular weight of 100,000 to 150,000 that are primarily composed of alkyl esters of methacrylic acid, crosslinked poly-N-vinyl carboxylic acid amides, benzylidene sorbitol and its derivatives, benzylidene xylitol and its derivatives, alkali-thickening acrylic resins, crosslinked acrylic acid polymers, inorganic fine particles, nonionic surfactants with an HLB value of 8 to 12, metal salts and amine salts of dialkyl sulfosuccinic acid, and the like.

[0128] The shear thinning agents may be used alone or in combination of two or more.

[0129] Examples of water-soluble polysaccharides include xanthan gum, welan gum, zeta sea gum, diutan gum, macrophomopsis gum, succinoglycan (average molecular weight: approximately 1 to 8 million), which is an organic acid-modified heteropolysaccharide whose constituent monosaccharides are glucose and galactose, guar gum, locust bean gum and its derivatives, hydroxyethyl cellulose, alkyl alginates, glucomannan, and carbohydrates having gelling ability extracted from seaweed, such as agar and carrageenan.

[0130] The ink composition according to the present invention may contain a polymer flocculant. The polymer flocculant creates a gentle bridging action between pigments, including the microencapsulated pigment, and the pigments form gentle aggregates via the polymer flocculant, thereby preventing the pigments from directly flocculating with each other and improving the dispersion stability of the pigments. The polymer flocculant has the effect of suppressing the settling of the pigment in the ink composition in the capillary gaps of the ink occluder provided in the marking pen described below. Therefore, the ink composition containing the polymer flocculant is preferably applied to a marking pen provided with an ink occluder.

[0131] When the ink composition according to the present invention contains a polymer flocculant, the content of the polymer flocculant relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 0.05 to 1 mass %.

[0132] Examples of the polymer flocculant include polyvinylpyrrolidone, polyethylene oxide, and water-soluble polysaccharides.

[0133] Examples of water-soluble polysaccharides include tragacanth gum, guar gum, pullulan, cyclodextrin, water-soluble cellulose derivatives, and the like.

[0134] Examples of the water-soluble cellulose derivatives include carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, hydroxypropyl cellulose, hydroxyethyl methyl cellulose, and hydroxypropyl methyl cellulose.

[0135] The ink composition of the present invention may contain a dispersant to enhance the dispersibility of the pigment. A polymer flocculant and a dispersant may also be used in combination. When both a polymer flocculant and a dispersant are used in combination, the dispersibility of the loose aggregates formed through the polymer flocculant can be improved.

[0136] Examples of dispersants include synthetic resins such as polyvinylpyrrolidone, polyvinyl butyral, polyvinyl ether, styrene-maleic acid copolymer, ketone resin, hydroxyethyl cellulose and its derivatives, styrene-acrylic acid copolymer, acrylic polymers, PO·EO adducts, and polyester amine oligomers. When the pigment contains a microencapsulated pigment, the dispersant is preferably an acrylic polymer dispersant, more preferably an acrylic polymer dispersant having a carboxy group, and even more preferably an acrylic polymer dispersant having a comb-shaped structure and having a carboxy group in the side chain, since the dispersant has excellent dispersibility of the microencapsulated pigment. Particularly preferred is an acrylic polymer dispersant having a comb-shaped structure and having multiple carboxy groups in the side chain, and a specific example of this is Solsperse 43000, a product of Lubrizol Japan Co., Ltd.

[0137] When the ink composition according to the present invention contains a dispersant, the content of the dispersant relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 0.01 to 2 mass%, more preferably 0.1 to 1.5 mass%. If the content of the dispersant exceeds 2 mass%, the pigment is likely to settle or float when subjected to external vibration, etc. On the other hand, if the content is less than 0.01 mass%, the effect of improving dispersibility is difficult to achieve.

[0138] The ink composition of the present invention may contain a surfactant, which allows the surface tension of the ink composition to be adjusted within an appropriate range. The surfactant may be a nonionic surfactant, an anionic surfactant, a cationic surfactant, an amphoteric surfactant, or the like, and any of them may be suitably used.

[0139] Examples of the surfactant include a phosphate-based surfactant, a silicone-based surfactant, a surfactant having an acetylene bond in its structure, a fluorine-based surfactant, and the like. These surfactants are appropriately selected depending on the components and applications of the ink composition.

[0140] When the ink composition according to the present invention contains a surfactant, the content of the surfactant relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 0.01 to 2 mass%, more preferably 0.05 to 1 mass%.

[0141] The ink composition of the present invention can be blended with a pH adjuster to adjust the pH of the ink composition to an appropriate range. As the pH adjuster, various acidic substances and basic substances can be used.

[0142] Examples of acidic substances include hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid, carbonic acid, boric acid, lactic acid, citric acid, tartaric acid, and malic acid.

[0143] Examples of basic substances include ammonia, sodium carbonate, sodium hydrogen phosphate, potassium hydrogen phosphate, sodium hydroxide, potassium hydroxide, sodium acetate, etc., and also alkanolamines such as monoethanolamine, diethanolamine, triethanolamine, etc. can be used.

[0144] When the ink composition according to the present invention contains a pH adjuster, the content of the pH adjuster relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 0.1 to 5 mass%, more preferably 0.5 to 2 mass%.

[0145] The ink composition according to the present invention can contain water-soluble resins such as alkyd resins, acrylic resins, styrene-maleic acid copolymers, cellulose derivatives, polyvinylpyrrolidone, polyvinyl alcohol, and dextrin, which can impart adhesion and viscosity to the paper surface. The water-soluble resins can be used alone or in combination of two or more.

[0146] When the ink composition according to the present invention contains a water-soluble resin, the content of the water-soluble resin relative to the total mass of the ink composition is not particularly limited, but is preferably in the range of 1 to 30 mass%, more preferably 1 to 10 mass%.

[0147] When the ink composition according to the present invention is used in a writing instrument (ballpoint pen) equipped with a ballpoint pen tip, the ink composition may also be blended with a lubricant. The lubricant improves the lubricity between the ball seat provided inside the tip body and the ball provided at the front end of the tip body, making it possible to easily prevent wear of the ball seat and improve the writing feel.

[0148] Examples of lubricants include higher fatty acids such as oleic acid; nonionic surfactants having a long-chain alkyl group; polyether-modified silicone oils; thiophosphite triesters such as thiophosphite tri(alkoxycarbonylmethyl ester) and thiophosphite tri(alkoxycarbonylethyl ester); phosphate surfactants such as polyoxyethylene alkyl ether or polyoxyethylene alkylaryl ether phosphate monoesters, polyoxyethylene alkyl ether or polyoxyethylene alkylaryl ether phosphate diesters, or metal salts, ammonium salts, amine salts, and alkanolamine salts of these phosphate esters.

[0149] The ink composition according to the present invention may further contain various additives as required.

[0150] Examples of the additives include rust inhibitors, preservatives, antifungal agents, air bubble absorbers, wetting agents, defoamers, antioxidants, and ultraviolet absorbing agents.

[0151] Examples of the rust inhibitor include dicyclohexylammonium nitrite, diisopropylammonium nitrite, and saponin.

[0152] Examples of the preservative or antifungal agent include carbolic acid, sodium salt of 1,2-benzothiazolin-3-one, sodium benzoate, sodium dehydroacetate, potassium sorbate, propyl paraoxybenzoate, 2,3,5,6-tetrachloro-4-(methylsulfonyl)pyridine, and the like.

[0153] Examples of air bubble absorbents include ascorbic acids, erythorbic acids, α-tocopherol, catechins, synthetic polyphenols, kojic acid, alkylhydroxylamines, oxime derivatives, α-glucosylrutin, α-lipoic acid, phosphonates, phosphinates, sulfites, sulfoxylates, dithionites, thiosulfates, and thiourea dioxide.

[0154] Examples of the wetting agent include reduced or non-reduced starch hydrolysates, disaccharides such as trehalose, oligosaccharides, sucrose, cyclodextrin, glucose, dextrin, sorbitol, mannitol, sodium pyrophosphate, and the like.

[0155] An example of the defoaming agent is dimethylpolysiloxane.

[0156] Examples of the antioxidant include dibutylhydroxytoluene, nordihydroxytoluene, flavonoids, butylhydroxyanisole, ascorbic acid derivatives, α-tocopherol, and catechins.

[0157] Examples of ultraviolet absorbents include 2-(2'-hydroxy-3'-tert-butyl-5'-methylphenyl)-5-chlorobenzotriazole, 2-(2'-hydroxy-5'-methylphenyl)benzotriazole, and p-benzoic acid-2-hydroxybenzophenone.

[0158] The method for producing the ink composition according to the present invention is not particularly limited, and any conventionally known method can be used. Specifically, the ink composition can be produced by stirring a mixture of the above-mentioned components with various stirrers such as a propeller stirrer, a homodisper, or a homomixer, or by dispersing the mixture with various dispersers such as a bead mill.

[0159] When the ink composition according to the present invention is used in a ballpoint pen, its viscosity is measured at a rotation speed of 1 rpm (shear rate of 3.84 sec) in an environment of 20° C. -1 When the viscosity is measured under the conditions described above, the ink composition can be more stably maintained, and therefore the following range is preferable. Specifically, the viscosity of the ink composition in this case is preferably in the range of 1 to 30,000 mPa s, and more preferably 100 to 25,000 mPa s. In addition, in an environment of 20°C, the rotation speed was 100 rpm (shear rate 384 sec -1 When the viscosity is measured under the conditions described above, the ink can be discharged well from the tip of the ballpoint pen, and therefore the viscosity is preferably in the following range. In particular, the viscosity of the ink composition in this case is preferably in the range of 1 to 15,000 mPa s, and more preferably in the range of 10 to 5,000 mPa s. By setting the viscosity of the ink composition within the above range, the stability of the ink composition and the free flowability of the ink composition within the mechanism of the ballpoint pen can be maintained at a high level.

[0160] The viscosity of the ink composition was measured using a rheometer (manufactured by TA Instruments, product name: Discovery HR-2, cone plate (diameter 40 mm, angle 1°)) at a rotation speed of 1 rpm (shear rate 3.84 sec -1 ), or rotation speed 100 rpm (shear rate 384 sec -1 ) values ​​measured under the conditions.

[0161] When the ink composition according to the present invention is used in a marking pen, the viscosity thereof is preferably in the following range since the stability and fluidity of the ink composition can be maintained at a high level when measured at a rotation speed of 50 rpm in an environment of 20° C. In detail, the viscosity of the ink composition in this case is preferably in the range of 1 to 30 mPa s, more preferably 1 to 20 mPa s, and even more preferably 1 to 10 mPa s.

[0162] The viscosity of the ink composition was measured using an E-type rotational viscometer (manufactured by Toki Sangyo Co., Ltd., product name: RE-85L, cone-type rotor: standard type (1°34′×R24)) by placing the ink composition in an environment of 20°C.

[0163] When the ink composition according to the present invention is used in a ballpoint pen or a marking pen, the pH thereof is preferably in the range of 4 to 10, more preferably 5 to 9. By keeping the pH within the above range, excessive viscosity increase and deterioration of the ink composition can be suppressed.

[0164] The pH of the ink composition was measured by placing the ink in an environment of 20° C. using a pH meter (manufactured by DKK-TOA Corp., product name: IM-40S).

[0165] When the ink composition according to the present invention is used in a ballpoint pen, the structure and shape of the ballpoint pen itself are not particularly limited, and the ink composition may be used, for example, by filling a ballpoint pen refill or ballpoint pen equipped with a ballpoint pen tip and an ink filling mechanism.

[0166] A ballpoint pen tip is composed of a tip body and a ball provided at the front end of the tip body. Examples of ballpoint pen tips include a tip in which a ball is held by a ball holding portion formed by pressing and deforming the vicinity of the tip of a metal pipe body inward from the outer surface, a tip in which a ball is held by a ball holding portion formed by cutting a metal material with a drill or the like, a tip in which a resin ball receiving seat is provided inside a metal or plastic tip body, and a tip in which the ball held by the tip is biased forward by a spring body.

[0167] The material of the tip body and the ball is not particularly limited, and examples thereof include cemented carbide (super hard), stainless steel, ruby, ceramic, resin, rubber, etc. Furthermore, the ball can be subjected to a surface treatment such as a DLC coating.

[0168] The diameter of the ball is generally in the range of 0.2 to 3 mm, preferably 0.2 to 2 mm, more preferably 0.2 to 1.5 mm, and further preferably 0.2 to 1 mm.

[0169] An example of the ink filling mechanism is an ink reservoir that can be directly filled with ink.

[0170] The ink reservoir may be, for example, a molded body made of a thermoplastic resin such as polyethylene, polypropylene, polyethylene terephthalate, or nylon, or a tubular body made of metal.

[0171] A ballpoint pen refill (hereinafter sometimes referred to as a "refill") can be formed by connecting a ballpoint pen tip directly or via a connecting member to an ink container and directly filling the ink container with ink. A ballpoint pen can be formed by storing this refill in a barrel.

[0172] The rear end of the ink reservoir is filled with an ink backflow prevention body, which may be a liquid plug or a solid plug.

[0173] The liquid plug is made of a non-volatile liquid and / or a difficult-to-volatile liquid, examples of which include petrolatum, spindle oil, castor oil, olive oil, refined mineral oil, liquid paraffin, polybutene, α-olefins, α-olefin oligomers or cooligomers, dimethyl silicone oil, methylphenyl silicone oil, amino-modified silicone oil, polyether-modified silicone oil, fatty acid-modified silicone oil, and the like. The non-volatile liquid and / or the hardly-volatile liquid can be used alone or in combination of two or more kinds.

[0174] It is preferable to add a thickener to the non-volatile liquid and / or the low-volatility liquid to thicken it to a suitable viscosity. Examples of thickeners include clay-based thickeners such as silica with a hydrophobic surface treatment, fine particle silica with a methylated surface, aluminum silicate, swellable mica, and hydrophobically treated bentonite or montmorillonite; fatty acid metal soaps such as magnesium stearate, calcium stearate, aluminum stearate, and zinc stearate; dextrin-based compounds such as tribenzylidene sorbitol, fatty acid amides, amide-modified polyethylene wax, hydrogenated castor oil, and fatty acid dextrin; and cellulose-based compounds.

[0175] Examples of solid plugs include solid plugs made of polyethylene, polypropylene, polymethylpentene, and the like.

[0176] As the ink backflow preventer, the above-mentioned liquid plug and solid plug can be used in combination.

[0177] In addition, it is also possible to form a ballpoint pen equipped with a ballpoint pen tip and an ink filling mechanism by using the barrel itself as the ink filling mechanism, filling ink directly into the barrel, and attaching a ballpoint pen tip to the front end of the barrel.

[0178] When the ink filled in the ink filling mechanism has a low viscosity, a ballpoint pen having a ballpoint pen tip and an ink filling mechanism may further include an ink supply mechanism for supplying the ink filled in the ink filling mechanism to the pen tip.

[0179] The ink supply mechanism is not particularly limited, and examples include (1) a mechanism that has an ink guide core made of a fiber bundle or the like as an ink flow rate regulator and supplies ink to the pen tip through this, (2) a mechanism that has a comb-shaped ink flow rate regulator and supplies ink to the pen tip through this, and (3) a mechanism that supplies ink to the pen tip through a pen core consisting of a number of disks arranged in parallel with comb-shaped intervals, with slit-shaped ink guide grooves running vertically through the disks in the axial direction and wider air vent grooves than the grooves, and with an ink guide core located in the axial center to guide ink from the ink filling mechanism to the pen tip.

[0180] The material of the pen core is not particularly limited as long as it is a synthetic resin that can be injection molded into a multiplicity of discs in a comb-like structure. Examples of synthetic resins include general-purpose polycarbonate, polypropylene, polyethylene, acrylonitrile-butadiene-styrene copolymer (ABS resin), etc. In particular, acrylonitrile-butadiene-styrene copolymer (ABS resin) is preferably used because it has high moldability and is easy to obtain pen core performance.

[0181] When a ballpoint pen is provided with the ink supply mechanism, the ink filling mechanism may be an ink occlusion body that can be filled with ink, in addition to the ink reservoir and barrel described above.

[0182] The ink occlusion body is a fiber bundle in which crimped fibers are bundled in the longitudinal direction, and is configured so that it is contained within a covering body such as a plastic cylinder or film, and the porosity is adjusted to be in the range of approximately 40 to 90%.

[0183] A ballpoint pen refill including a ballpoint pen tip, an ink filling mechanism, and an ink supply mechanism can be formed by accommodating an ink occluder impregnated with ink in an ink container, providing an ink supply mechanism at the front end of the ink container so as to connect to the ink occluder, and connecting a ballpoint pen tip to the ink supply mechanism directly or via a connecting member. Alternatively, a ballpoint pen refill can be formed by accommodating an ink occluder impregnated with ink in an ink container, providing an ink supply mechanism inside the ink container so as to connect to the ink occluder, and connecting a ballpoint pen tip to the ink container directly or via a connecting member.

[0184] Specific examples of the configuration of a ballpoint pen containing the ink composition according to the present invention include: (1) a ballpoint pen having an ink reservoir filled with ink within a barrel, to which a ballpoint pen tip is connected either directly or via a connecting member, and in which an ink backflow preventer is filled at the end face of the ink; (2) a ballpoint pen in which the barrel is directly filled with ink, and which is provided with a mechanism for supplying ink to the pen tip by using a comb-shaped ink flow regulator or an ink guide core made of a fiber bundle or the like as an ink flow regulator; (3) a ballpoint pen in which the barrel is directly filled with ink, and which is provided with a mechanism for supplying ink to the pen tip via the above-mentioned pen core; and (4) a ballpoint pen in which the barrel contains an ink occlusion body made of a fiber bundle impregnated with ink, and which is provided with a mechanism for supplying ink to the pen tip by using an ink guide core made of a fiber bundle or the like as an ink flow regulator.

[0185] When the ink composition according to the present invention is used in a marking pen, the structure and shape of the marking pen itself are not particularly limited, and the ink composition may be used, for example, by filling a marking pen refill or a marking pen equipped with a marking pen tip and an ink filling mechanism.

[0186] Examples of marking pen tips include conventional porous members with interconnected pores, such as resin-processed fibers, fused heat-fusible fibers, and felt, which have a porosity selected from a range of approximately 30 to 70%, or extrusion-molded synthetic resin bodies with multiple ink outlet holes extending in the axial direction, one end of which can be processed into a bullet shape, rectangular shape, chisel shape, or other shape suited to the purpose for which it is used.

[0187] An example of the ink filling mechanism is an ink occlusion body that can be filled with ink. The ink occlusion body is a fiber bundle in which crimped fibers are bundled in the longitudinal direction, and is placed inside a covering such as a plastic cylinder or film, with the porosity adjusted to the range of approximately 40 to 90%.

[0188] A marking pen can be formed by housing an ink occlusion body impregnated with ink inside the barrel and connecting a marking pen tip to the barrel directly or via a connecting member so as to connect to the ink occlusion body.

[0189] Also, a marking pen refill (hereinafter sometimes referred to as a "refill") can be formed by storing an ink occlusion body impregnated with ink in an ink container and connecting a marking pen tip to the ink container directly or via a connecting member so as to be connected to the ink occlusion body. A marking pen can be formed by storing this refill in a barrel.

[0190] The ink reservoir may be, for example, a molded body made of a thermoplastic resin such as polyethylene, polypropylene, polyethylene terephthalate, or nylon, or a tubular body made of metal.

[0191] A marking pen equipped with a marking pen tip and an ink filling mechanism may further include an ink supply mechanism for supplying the ink composition filled in the ink filling mechanism to the pen tip.

[0192] The ink supply mechanism is not particularly limited, and examples thereof include, in addition to the ink supply mechanism provided in the ballpoint pen described above, (4) a mechanism provided with an ink flow rate regulator using a valve mechanism, which supplies ink to the pen tip by opening the valve.

[0193] The valve mechanism can be of the conventional, general-purpose pumping type that opens when the tip is pressed, and is preferably set to a spring pressure that can be pressed and opened by the pressure of the writing pen.

[0194] When the marking pen is provided with an ink supply mechanism, the ink filling mechanism may be an ink reservoir that can be filled directly with ink, in addition to the ink occlusion body described above. Also, the barrel itself may serve as the ink filling mechanism, and ink may be filled directly.

[0195] A marking pen refill including a marking pen tip, an ink filling mechanism, and an ink supply mechanism can be formed by accommodating an ink occluder impregnated with ink in an ink container, providing an ink supply mechanism at the front end of the ink container so as to connect to the ink occluder, and connecting a marking pen tip to the ink supply mechanism directly or via a connecting member. Alternatively, a marking pen refill can be formed by accommodating an ink occluder impregnated with ink in an ink container, providing an ink supply mechanism inside the ink container so as to connect to the ink occluder, and connecting a marking pen tip to the ink container so as to connect to the ink supply mechanism directly or via a connecting member.

[0196] Specific configurations of marking pens containing the ink composition of the present invention include: (1) a marking pen in which an ink occlusion body made of a fiber bundle impregnated with ink is contained in a barrel, and a marking pen tip made of a fiber processed body or a resin molded body having capillary gaps is connected to the barrel directly or via a connecting member so that the ink occlusion body and the tip are connected; (2) a marking pen in which ink is directly filled in the barrel, and which is provided with a mechanism for supplying ink to the pen tip by using a comb-shaped ink flow regulator or an ink guide core made of a fiber bundle or the like as an ink flow regulator; (3) a marking pen in which ink is directly filled in the barrel, and a comb-shaped ink flow regulator or an ink guide core made of a fiber bundle or the like as an ink flow regulator; Examples of such marking pens include a marking pen that is filled with ink and is equipped with a mechanism for supplying ink to the pen tip via the pen core; (4) a marking pen that is equipped with a tip and an ink container via a valve mechanism that opens when the tip is pressed, and in which ink is directly filled into the ink container; and (5) a marking pen that has an ink container that contains an ink occlusion body made of a fiber bundle impregnated with ink within a barrel, and a marking pen tip made of a fiber processed body or a resin molded body with capillary gaps formed therein is connected to the ink container directly or via a connecting member so that the ink occlusion body and the tip are connected.

[0197] When the ballpoint pen or marking pen according to the present invention is one in which ink is directly filled, an agitator such as an agitating ball for agitating the ink can be built into the ink reservoir or barrel in which the ink is filled in order to facilitate redispersion of the pigment, including the microencapsulated pigment. Examples of the shape of the agitator include a spherical body and a rod-shaped body. The material of the agitator is not particularly limited, and examples thereof include metal, ceramic, resin, and glass.

[0198] The writing instrument according to the present invention, such as a ballpoint pen or a marking pen, may be in the form of an ink cartridge as a removable structure. In this case, after the ink contained in the ink cartridge of the writing instrument is used up, the writing instrument can be used again by replacing it with a new ink cartridge.

[0199] As the ink cartridge, one that also serves as the barrel that constitutes the writing instrument by connecting it to the writing instrument body, or one that covers and protects the barrel (rear barrel) after connecting it to the writing instrument body, is used. In addition to being used alone, the latter may be either one in which the writing instrument body and the ink cartridge are connected in the writing instrument before use, or one that is stored in the barrel in a disconnected state so that the user of the writing instrument can connect the ink cartridge in the barrel when using it and start using it.

[0200] In the writing instrument such as the ballpoint pen or marking pen according to the present invention, a cap is provided to cover the pen tip (writing tip) to make it a capped writing instrument, thereby preventing the writing tip from being contaminated or damaged.

[0201] In addition, in writing instruments such as ballpoint pens or marking pens in which a refill is stored inside the barrel, a retractable mechanism can be provided inside the barrel that allows the writing tip to protrude and retract from the barrel, making the writing instrument a retractable type, thereby preventing the writing tip from being contaminated or damaged.

[0202] Any retractable writing instrument can be used as long as the writing tip is housed within a barrel with the writing tip exposed to the outside air and the retractable mechanism is activated to cause the writing tip to protrude from the barrel opening. It may also be a composite type retractable writing instrument in which a plurality of refills are housed within the barrel, and the writing tip of any one of the refills is caused to protrude and retract from the barrel opening by operation of a retraction mechanism.

[0203] Examples of the retraction mechanism include: (1) a side-slide type retraction mechanism in which an operating part (clip) that can move in the front-rear direction protrudes radially outward from the rear side wall of the barrel, and the operating part is slid forward to cause the writing tip to appear and disappear from the front opening of the barrel; (2) a rear-end knock type retraction mechanism in which an operating part provided at the rear end of the barrel is pressed forward to cause the writing tip to appear and disappear from the front opening of the barrel; (3) a side-knock type retraction mechanism in which an operating part protruding from the outer surface of the side wall of the barrel is pressed radially inward to cause the writing tip to appear and disappear from the front opening of the barrel; and (4) a rotating type retraction mechanism in which an operating part at the rear of the barrel is rotated to cause the writing tip to appear and disappear from the front opening of the barrel.

[0204] The configuration of ballpoint pens and marking pens is not limited to the configurations described above, and they may be equipped with tips of different shapes, or with tips that dispense ink of different tones or hues, or they may be composite writing instruments (double-headed, retractable tip, etc.) that are equipped with tips of different shapes and dispense ink of different tones or hues.

[0205] A preferred writing instrument according to the present invention is a retractable writing instrument (a retractable ballpoint pen or a retractable marking pen) in which a refill is accommodated within a barrel. Generally, the writing tip of a retractable writing instrument is always exposed, and the writing tip is prone to drying out and causing writing defects. However, the ink composition of the present invention has excellent resistance to drying up and can inhibit drying out of the writing tip, making writing defects such as smudging less likely to occur, and is therefore suitable for use in retractable writing instruments.

[0206] When a reversible thermochromic composition, a reversible thermochromic microencapsulated pigment, or a reversible thermochromic resin particle is used as a colorant, the writing made on the surface to be written on using a writing instrument containing the ink composition of the present invention can be discolored by rubbing with a finger or by using a heating or cooling tool.

[0207] Examples of the heating device include an electrically-heated discoloring device equipped with a resistive heating element such as a PTC element, a heat discoloring device filled with a medium such as hot water, a heat discoloring device using steam or laser light, and the application of a hair dryer. However, friction members and friction bodies are preferred because they can change color in a simple manner.

[0208] Examples of cooling devices include electrochemically-induced color-changing devices using a Peltier element, color-changing devices filled with a refrigerant such as cold water or ice chips, cooling agents, refrigerators, freezers, and the like.

[0209] As the friction member and friction body, an elastic body such as an elastomer or a plastic foam, which has a high elastic feel and can generate appropriate friction and generate frictional heat when rubbed, is preferable, but a plastic molded body, stone, wood, metal, cloth, etc. can also be used.

[0210] Although a general eraser used for erasing pencil marks may be used to rub the marks, the rubbed marks will generate eraser dust. Therefore, the above-mentioned friction member and friction body which generate almost no eraser dust are preferably used.

[0211] Examples of the material of the friction member and the friction body include silicone resin, styrene-ethylene-butadiene-styrene block copolymer (SEBS resin), etc. Silicone resin is likely to adhere to the part erased by rubbing, and handwriting tends to be repelled when writing is repeated, so SEBS resin is more preferably used.

[0212] The friction member or friction body may be a member of any shape that is separate from the writing instrument, but by providing it on the writing instrument, the writing instrument can be made highly portable. Also, a writing instrument set can be obtained by combining a writing instrument with a friction member or friction body of any shape that is separate from the writing instrument.

[0213] In the case of a writing instrument with a cap, the location where the friction member or friction body is provided is not particularly limited. For example, the cap itself may be formed from a friction member, the barrel itself may be formed from a friction member, and in the case where a clip is provided, the clip itself may be formed from a friction member, or the friction member or friction body may be provided at the tip (top) of the cap or the rear end of the barrel (the part where the writing tip is not provided), etc.

[0214] In the case of a writing instrument equipped with a retractable mechanism, the location where the friction member or friction body is provided is not particularly limited, and for example, the barrel itself may be formed from a friction member, and if a clip is further provided, the clip itself may be formed from a friction member, or the friction member or friction body may be provided near the opening of the barrel, at the rear end of the barrel (the portion where the writing tip is not provided), or at the knock portion. EXAMPLES

[0215] The following examples are given, but the present invention is not limited thereto. In the examples, "parts" refers to "parts by mass" unless otherwise specified.

[0216] Preparation of deep eutectic solvent A 46.25 parts of urea as a hydrogen bond donor compound and 53.75 parts of choline chloride as a hydrogen bond acceptor compound were placed in a beaker, and the contents of the beaker were stirred and mixed with a stirrer for 30 minutes while heating at 100°C to prepare deep eutectic solvent A. The molar ratio of the hydrogen bond donor compound to the hydrogen bond acceptor compound was 2:1.

[0217] Preparation of deep eutectic solvents B to D Deep eutectic solvents B to D were prepared in the same manner as deep eutectic solvent A, except that the types and amounts of the hydrogen bond donor compound and hydrogen bond acceptor compound were changed to those shown in Table 1 below. The molar ratios of the hydrogen bond donor compound:hydrogen bond acceptor compound are shown in Table 1 below. The numbers in parentheses in the table indicate parts by weight.

[0218] [Table 1]

[0219] Preparation of reversible thermochromic microencapsulated pigments A reversible thermochromic composition consisting of 3 parts of 3',6'-bis[phenyl(3-methylphenyl)amino]spiro[isobenzofuran-1(3H),9'-[9H]xanthene]-3-one as component (A), 3 parts of 1,1-bis(4-hydroxyphenyl)-2-ethylhexane and 5 parts of 2,2-bis(4-hydroxyphenyl)hexafluoropropane as component (B), and 50 parts of 4-benzyloxyphenylethyl caprate as component (C) was added to a mixed solution consisting of 35 parts of aromatic isocyanate prepolymer as a wall film material and 40 parts of cosolvent, and then emulsified and dispersed in an 8% polyvinyl alcohol aqueous solution. After continuing to stir while heating, 2.5 parts of water-soluble aliphatic modified amine were added and further stirring was continued to prepare a microcapsule dispersion. A reversible thermochromic microcapsule pigment with an average particle size of 1.9 μm was obtained from the microcapsule dispersion by centrifugation. The microencapsulated pigment had a complete color development temperature t1 of -20°C and a complete decolorization temperature t4 of 60°C, and reversibly changed from blue to colorless with temperature change.

[0220] Example 1 Preparation of ink composition for writing instruments A writing instrument ink composition was prepared by mixing 10 parts of a blue dye (manufactured by Sumitomo Chemical Co., Ltd., product name: Acid Blue PH) and 90 parts of deep eutectic solvent A.

[0221] Creation of writing implements The ink composition was filled by suction into an ink container made of polypropylene, and then connected to a ballpoint pen tip having a 0.5 mm diameter ball made of cemented carbide (super hard) at the tip via a resin holder. Next, a viscoelastic ink backflow prevention body (liquid plug) mainly composed of polybutene was filled into the rear end of the ink container, and a tail plug was fitted to the rear of the pipe, followed by degassing by centrifugation to obtain a ballpoint pen refill. Next, the refill was incorporated into a barrel to prepare a writing instrument (ballpoint pen). In the above-mentioned ballpoint pen, the tip provided in the ballpoint pen refill is stored inside the barrel while being exposed to the outside air, and a rear-end knock-type protruding / retracting mechanism is provided in which the tip protrudes from the front end opening of the barrel by pressing forward an operating part provided at the rear end of the barrel.

[0222] Examples 2 to 7 and Comparative Examples 1 and 2 Preparation of ink composition for writing instruments The writing instrument ink compositions of Examples 2 to 7 and Comparative Examples 1 and 2 were prepared in the same manner as Example 1, except that the types and amounts of the materials to be blended were changed to those shown in Table 2 below.

[0223] Creation of writing implements The writing instruments (ballpoint pens) of Examples 2 to 7 and Comparative Examples 1 and 2 were produced in the same manner as in Example 1.

[0224] Example 8 Preparation of ink composition for writing instruments A writing instrument ink composition was prepared by mixing 3 parts of a blue dye (manufactured by Sumitomo Chemical Co., Ltd., product name: Acid Blue PH), 5 parts of deep eutectic solvent B, and 92 parts of water.

[0225] Creation of writing implements The above-mentioned ink composition was impregnated into an ink reservoir made of polyester sliver covered with a synthetic resin film, and the ink was housed in a barrel made of polypropylene. A resin-processed pen body (chisel type) made of polyester fiber was attached to the tip of the barrel via a resin holder, and a cap was attached to produce a writing instrument (marking pen).

[0226] Comparative Example 3 Preparation of ink composition for writing instruments The writing instrument ink composition of Comparative Example 3 was prepared in the same manner as in Example 8, except that the types and amounts of the materials to be blended were changed to those shown in Table 3 below.

[0227] Creation of writing implements The writing instrument (marking pen) of Comparative Example 3 was produced in the same manner as in Example 8.

[0228] [Table 2]

[0229] [Table 3]

[0230] The contents of the materials in Tables 2 and 3 are explained according to the note numbers. (1) Blue dye [Sumitomo Chemical Co., Ltd., product name: Acid Blue PH] (2) Black dye [manufactured by Orient Chemical Industries Co., Ltd., product name: Water Black R510] (3) Erythrosine [manufactured by Daiwa Chemical Industries, Ltd., product name: Food Red No. 3] (4) Carbon black [manufactured by Mitsubishi Chemical Corporation, product name: MA-100] (5) Reversible thermochromic microencapsulated pigment (6) Polyvinylpyrrolidone [manufactured by BASF, product name: Socalan K-30] (7) Nonionic surfactant [Nikko Chemicals Co., Ltd., product name: PBC-34] (8) Phosphate ester surfactant [manufactured by Daiichi Kogyo Seiyaku Co., Ltd., product name: Plysurf AL]

[0231] [Evaluation of initial writing performance] Using the ballpoint pens prepared in Examples 1 to 7 and Comparative Examples 1 and 2, 15 cm straight lines were handwritten in a direction parallel to the short side of an A4 size test paper (portrait) in a room temperature (20°C) environment, and five lines were written in this manner. In addition, using each marking pen prepared in Example 8 and Comparative Example 3, 15 cm straight lines were handwritten in a direction parallel to the short side of an A4 size test paper (portrait) in a room temperature (20°C) environment, with the wide surface of the pen body in close contact with the paper surface, and five lines were written in this manner. Note that writing paper A conforming to the old JIS P3201 was used as the test paper. The resulting handwriting was visually inspected and evaluated according to the following criteria. The evaluation results are shown in Tables 4 and 5 below, with ratings of "A" and "B" being acceptable. A: There was no smearing or smearing in the handwriting, and good handwriting was obtained. B: Some smearing was observed in the handwriting, but it was at a level that did not cause any practical problems. C: Numerous smudges were found in the handwriting.

[0232] [Evaluation of dry-up resistance] The writing tip of each ballpoint pen used in the above-mentioned writing test was left in a protruding state (knock-on state) and left to stand in a landscape position for one day in a room temperature (20°C) environment. After one day, a 15 cm straight line was handwritten in a room temperature (20°C) environment parallel to the short side of an A4 size test paper (portrait), and this was done for five lines. In addition, the cap of each marking pen used in the above-mentioned writing test was removed, and the pen was left to stand in a landscape position for 30 minutes in a room temperature (20°C) environment. After 30 minutes, a 15 cm straight line was handwritten in a room temperature (20°C) environment parallel to the short side of an A4 size test paper (portrait), with the wide surface of the pen body in close contact with the paper, and this was done for five lines. Note that writing paper A conforming to the old JIS P3201 was used as the test paper. The resulting handwriting was visually inspected and evaluated according to the following criteria. The evaluation results are shown in Tables 4 and 5 below, with ratings of "A" and "B" being acceptable. A: There was no smearing in the handwriting, and good handwriting similar to the initial handwriting was obtained. B: The handwriting was slightly blurred compared to the initial handwriting, but was at a level that did not cause any practical problems. C: Compared to the earlier handwriting, many smudges were found in the handwriting.

[0233] [Table 4]

[0234] [Table 5]

Claims

1. An ink composition for a writing instrument comprising at least a colorant and a deep eutectic solvent.

2. The ink composition of claim 1 , wherein the deep eutectic solvent comprises a compound having hydrogen bond donor properties and a compound having hydrogen bond acceptor properties.

3. 3. The ink composition according to claim 2, wherein the hydrogen bond donor compound is a compound selected from the group consisting of alcohols, sugars, organic acids, and nitrogen-containing compounds, and the hydrogen bond acceptor compound is a compound selected from the group consisting of non-metal salts, betaines, amino acids, polycarboxylic acids, and phosphate esters.

4. The ink composition of claim 1 further comprising water.

5. A writing instrument containing the ink composition according to any one of claims 1 to 4.

6. 6. The writing instrument of claim 5, wherein the writing instrument is a retractable writing instrument.